[CrOS MultiDevice] Remove unnecessary multi-device namespaces.

Several files which reside in the DeviceSync, MultiDeviceSetup, and
SecureChannel services were moved into those directories during the
multi-device directory cleanup. Before the move, these files used other
namespaces (e.g., cryptauth::*), so when they referenced multi-device
services, they namespaced types using chromeos::device_sync::*,
chromeos::multidevice_setup::*, and chromeos::secure_channel::*.

Now that these files reside in the services themselves and use the same
namespaces as the rest of the files in those directories, these
namespaces are no longer needed and cause unnecessary verbosity. This CL
removes them.

Bug: 912743
Change-Id: I50aee4767bb4e131e90e9ac7df9e77655a0b4ece
Reviewed-on: https://chromium-review.googlesource.com/c/1385108
Reviewed-by: Jeremy Klein <jlklein@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618013}
51 files changed